In general, fine patterns are formed in semiconductor device manufacturing processes using photolithography. A plurality of photomasks (to be referred to as “transfer masks”) is normally used to form these fine patterns. These transfer masks typically have a fine pattern composed of a metal thin film and the like provided on a transparent glass substrate, and photolithography is also used in the manufacturing of these transfer masks.
A mask blank is used in the manufacturing of transfer masks by photolithography. In general, mask blanks are manufactured by forming a thin film on the main surface of a transparent substrate composed of synthetic quartz and the like by a sputtering method. The thin films of these mask blanks tend to be formed on the main surface of a substrate while retaining internal stress.
The main surface of a mask blank is required to have a high flatness. The main surface of the transparent substrate used for the mask blank substrate is also required to have a high flatness. Consequently, the main surface of a mask blank substrate is subjected to processing such as grinding or polishing. However, in the case of having formed a thin film retaining a large amount of internal stress on a transparent substrate having a main surface having a high flatness in this manner, the main surface of the transparent substrate ends up being deformed resulting in the problem of exacerbation of the flatness of the main surface of the transparent substrate.
On the other hand, in the case of a thin film for forming a transfer pattern, a pattern is formed by removing a portion of the thin film (light-transmitting portion) by etching and the like. In the case the thin film retains a large amount of internal stress, when a portion of the thin film (portion serving as the light-transmitting portion) has been removed by etching and the like, the position of the pattern on the transparent substrate ends up moving due to the thin film being released from the internal stress (pattern shifting).
The requirements regarding pattern positional accuracy placed on transfer masks have become even more severe in recent years. The allowed amount of this positional shift has become extremely small in the manufacturing of photo masks applied in double patterning technology in particular.
In double patterning technology, an extremely fine pattern formed on a semiconductor device is divided into two comparatively sparse patterns. Two transfer masks are then fabricated having each of these divided patterns, and patterns are exposed and transferred on the semiconductor device using these two transfer masks. As a result, extremely fine patterns can be formed on a semiconductor device. However, in the case of double patterning technology, if the amount of the positional shift from a design pattern of the patterns formed on the two transfer masks is large, when the patterns are exposed and transferred on the semiconductor device using the two transfer masks, the patterns may end up being formed in a state in which the patterns contain disconnections or short circuits.
In order to solve the aforementioned problem, research has previously been conducted on technologies for reducing the internal stress of mask blank thin films.
For example, Patent Literature 1 describes a method for reducing internal stress of a thin film that comprises forming a thin film on a transparent substrate by a sputtering method followed by carrying out heat treatment on the thin film at a temperature of not less than 150° C. Patent Literature 2 describes a method for irradiating a thin film formed on a transparent substrate with a high energy beam using a flash lamp.
However, as is described in Patent Literature 3, in the case of a method in which a thin film is irradiated with a high energy beam using a flash lamp, the high energy beam ends up having a considerable effect on the glass substrate depending on the exposure level, and this was determined to result in the problem of deformation of the shape of the main surface of synthetic quartz glass substrates.